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The Unusual Absorption Line Spectrum of Quasar SDSS J2220+0109
CHINESE ASTRONOMY AND ASTROPHYSICS
ELSEVIER
Chinese Astronomy and Astrophysics 37 (2013) 17–27
The Unusual Absorption Line Spectrum of Quasar SDSS J2220+0109 ZHOU Hong-yan1,2
JI Tuo1,2 1
WANG Ting-gui1
WANG Hui-yuan2
University of Science and Technology of China, Hefei 230026 2 Polar Research Institute of China, Shanghai 200136
Abstract We report the Balmer broad absorption lines (BALs) in the quasar SDSS J2220+0109 discovered from the SDSS data, and present a detailed analysis of the peculiar absorption line spectrum, including the He I* multiplet at λλ3189, 3889 arising from the metastable 23 s-state helium and the Balmer Hα and Hβ lines from the excited hydrogen H I of n=2 level, which are rarely seen in quasar spectra, as well as many absorption lines arising from the excited Fe II* of the levels 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 in the wavelength range 3100∼3300 ˚ A. Ca II H, K absorption line doublets also clearly appear in the SDSS spectrum. All absorption lines show a similar blueshifted velocity structure of Δv ≈ −1500 ∼ 0 km·s−1 relative to the quasar’s systematic redshift determined from the emission lines. Detailed analysis suggests that the Balmer absorption lines should arise from the partially ionized region with a column density of NH I ≈ 1021 cm−2 for an electron density of ne ∼ 106 cm−3 ; and that the hydrogen n=2 level may be populated via collisional excitation with Lyα pumping. In addition to its peculiar absorption features, SDSS J2220+0109 also shows unusual Fe II emission lines very likely originated from the low-density gas of ne ≈ 106 cm−3 . Key words: quasar: absorption lines—quasar: emission lines—quasar: individual: SDSS J2220+0109
Received 2011–12–13; revised version 2012–02–21 A translation of Acta Astron. Sin. Vol. 53, No.5, pp. 357–368, 2012 [email protected]
0275-1062/13/$-see front front mattermatter © 2013 Elsevier All rights reserved. c 2013 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2013.01.003 PII:
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1. INTRODUCTION Outflows in AGNs are potentially important for the co-evolution of black holes and their host galaxies. First, the outflowing gases carry away angular moment and facilitate the accretion of gas into black holes; Second, they influence the star formation and chemical evolution in host galaxies by funneling metal-enriched circum-nuclear gases into the interstellar even intergalactic space. Furthermore, in a longer time scale, outflows blow away nuclear gases and adjust the co-growth of black holes and galaxy bulges, this may explain their observed mass relation[1−3] . Outflows often show themselves as blueshifted absorption line systems in quasar spectra. Based on the width Δv of absorption lines, they mainly fall into 3 categories[4]: (1) broad absorption lines (BALs) with Δv ≥ 2000 km s−1 ; (2) narrow absorption lines (NALs) with Δv ≤ 500 km s−1 ; (3) mini-BALs with Δv in between. BALs are only seen in 15% of optically selected quasars. Based on the degree of ionization of absorption gases, BALs can be classified into 3 categories: (1) high-ionization broad absorption lines (HiBAL), i.e., the absorption lines arising generally from the absorption of C IV, Si IV and N V; (2) lowionization broad absorption-lines (LoBALs), about 15% of BAL quasars show absorption lines from low-ionization species such as Al III, Al II and Mg II, besides HiBALs; (3) iron low-ionization BALs (FeLoBALs), a minority (another 15%) of LoBAL quasars exhibit additional absorption of Fe II and Fe III at both ground and excited levels. Hydrogen Balmer absorption lines and He I* absorption lines are even rarer than FeLoBALs. The former arise from the absorption of hydrogen atoms excited from the n=2 level and are only detected in six quasars, including NGC 4151[5], SDSS J083942.11+380526.3[6], SDSS J125942.80+121312.6[7], SDSS J102839.11+450009.4[8], SDSS J172341.10+555340.5[9], and LBQS J1209+1633[10]. The latter arise from the absorption of photons by He I* of metastable 23 s-state. The population of He I* 23 s-level mainly comes from the recombination of He+ with electrons. Transitions from this level will generate a series of absorption lines with wavelengths ranging from near-infrared to ultraviolet, including He I* λλ10830, 3889, 3189, 2946, 2830, 2764. To our knowledge, He I* absorption lines are only detected in 5 quasars, including Mrk 231[11,12] , NGC 4151[5,13], Q 2359-1241[14], SDSS J102839.11+450009.42008 [8] and FBQS J1151+3822[15]. Both Balmer and He I* absorption lines are detected in NGC 4151 and SDSS J102839.11+450009.4, this may indicate that a certain relation may exist between the two kinds of lines in spite of the small sample sizes of the two. Another interesting fact is that two out of six Balmer absorption quasars show absorption lines arising from the excited Fe II at the same time. The non-detection of Fe II* absorption lines in the other two quasars may be due to either the low-SNR spectrum (SDSS J102839.11+450009.4) or the limited wavelength coverage (LBQS J1209+1633). It is probable that these two absorption phenomena are also closely related. If this is the case, these quasars will be valuable to the study of quasar outflows. Fe II* absorption lines can be used to determine the density of the absorption gas[16], while He I* absorption lines can help to understand the ionization state of the gas[14,17,18]. The combination of the two diagnostics can put constraints on the geometry and physical conditions of outflows. Discovery of more Balmer, He I* or Fe II* absorption line quasars can help to understand the relation between these absorption line systems and the possible origins of these systems. In this paper, we report the unusual absorption line quasar SDSS J222024.59+010931.2
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(hereafter SDSS J2220+0109) as reveled from the SDSS data. In its spectrum, we have detected simultaneously the absorption lines including Hα, Hβ, He I* λλ3889, 3189, Ca II H, K and Fe II* absorption lines arising from the levels of wave-numbers 7 955 cm−1 , 13474 cm−1 and 13673 cm−1 . We analyze the SDSS spectrum and measure the parameters of absorption lines in Section 2; we discuss the population mechanism of the absorption species and the physical conditions in the gas in Section 3; finally we discuss the implications of the discovery and make a summary in Section 4.
2. ANALYSIS OF SDSS SPECTRUM We searched systematically for the He I* and Balmer absorption lines in SDSS DR7 quasars and found about 200 quasars showing He I* absorption and about 20 showing Balmer absorption lines. SDSS J2220+0109 is the brightest among them with the I-band magnitude of (16.292±0.015) mag. The spectrum of this object was taken by SDSS 2.5 m telescope on 2001-08-19. The redshift re-measured on the peak of [O III] emission line is z=0.2122±0.0025. The spectrum was corrected for a Galactic extinction of E(B-V)=0.0632 and brought to the rest frame of the quasar before further analysis. The spectrum covers 3140∼7600 ˚ A in the rest frame. Balmer absorption lines sit on the top of Hα and Hβ emission lines, it makes their measurement depend on that of emission lines. On the other hand, the UV absorption lines are free of this problem. Thus different methods were adopted for the analysis of the optical spectrum and UV spectrum. 2.1 Optical Spectrum Because Balmer absorption lines are superposed on the broad Balmer emission lines, in order to obtain the normalized absorption spectrum and measure the parameters of absorption lines, the unabsorbed continuum and broad emission lines must be recovered at first. Generally, they are fitted by a power-law continuum and a template of broadened Fe II emission lines, in addition to one or more Gaussians for each emission line. Optical Fe II templates are usually built from the prototype narrow-line Seyfert 1 galaxy (NLS1) I Zw 1. The one built by Boroson et al.[19] (BG92) only includes broad Fe II lines. However, we note that there are obvious narrow emission lines near 5170 ˚ A in the spectrum of SDSS J2220+0109. Thus we adopt the Fe II template built by V´eron-Cetty et al. [20] (VJV04), which includes both broad and narrow Fe II emission lines. We use 4 Gaussians for the broad emission lines in the spectral range, including Hα, Hβ and Hγ; each narrow emission line in the spectral range is modeled with one Gaussian, and the redshifts of the narrow lines are assumed to be the same. We also fix the doublet ratio of [O III] λλ4959, 5007 to their theoretical value 1:3, the flux of narrow Hβ line is assumed to be 10% of the value of [O III] λ5007. All the above components are jointly used to fit the SDSS spectrum in the range of 4000∼7500 ˚ A, and the pixels affected by absorption lines are masked before fitting. The best-fit parameters are achieved by the minimization of χ2 . In the lower panel of Figure 1, we show the fitted result in the Hα region. The result agrees well with the SDSS spectrum. In the upper panel of Figure 1, we show the fitted result around Hβ and Hγ. The residual spectrum is shifted downward arbitrarily for clarity. There are clear emission peaks in the residual spectrum, especially on the red wing of Hβ emission line. The residual spectrum is very similar to the Fe II spectrum emitted by low-density (ne ≈ 106 cm−3 ) gas
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fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
as calculated by Bruhweiler et al. [21] The analysis of low-density Fe II emission is beyond the scope of this paper, thus we only show the calculated low-density Fe II spectrum for comparison in Figure 1.
Fig. 1
60 40 20 0 −20 4200
4400
4600
4800 5000 Wavelength / Å
5200
5400
5600
6600
6800
7000
200 SDSS spectrum
150
Best fit Residual
100
Bruhweiler’s model
50 0 −50 5600
5800
6000
6200 6400 Wavelength / Å
The fitted result of optical spectrum (4000∼7000 ˚ A)
Although the fitting of Hα region is satisfactory, the pollution of extra Fe II emission around Hβ and the weakness of the Hβ absorption line make the normalization of absorption lines very sensitive to the details of fitting. To normalize the spectrum in an unified way, we use a two-order polynomial to fit the Hα and Hβ locally. First, we use isolated He I* λλ3889, 3189 lines to determine the velocity range affected by absorption lines (Δv ≈ −1500 ∼ 0 km· s−1 ); then we mask the corresponding pixels of Hα and Hβ absorption lines and perform locally the two-order polynomial fitting. In the following analysis, we will use this fitted result for normalizing the spectrum. 2.2 Ultraviolet Spectrum In the UV band, besides the power-law continuum, the AGN spectrum is composed of the high-order Balmer lines, Balmer continuum and Fe II emission lines, which mix into a pseudo-continuum. The spectrum can in principle be decomposed using multi-component fitting. Two UV (<3500 ˚ A) Fe II templates are usually used in spectral decomposition, they are from Vestergaard et al.[22] and Tsuzuki et al.[23] respectively. But the blue end of the SDSS spectrum only reaches 3200 ˚ A, and there seems to be extra narrow Fe II emission [8] multiplet found by Wang et al. around 3100∼3200 ˚ A, which are not accounted for in the above mentioned Fe II templates. All these factors make the decomposition technique unreliable, hence we also recover the unabsorbed spectrum by means of local polynomial regression as done for Balmer lines. We still mask the pixels affected by absorption lines as done for Hα and Hβ. Then we linearly interpolate on the masked pixels and perform locally the weighted scatter-plot smoothing (LOWESS) [24,25] algorithm on the interpolated spectrum. In the LOWESS process, a low-order polynomial is fitted to a fixed width of the spectrum iteratively by means of weighted least square. In each succeed iteration, the weights are chosen to be inversely proportional to the n-th power of the deviation between
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fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
the data and the currently fitted result. The iterations converge to our required absorptionfree spectrum. The algorithm is insensitive to narrow features including narrow emission and absorption lines and can maintain the broad features such as the Balmer continuum, highorder Balmer lines and Fe II emission lines. In Figure 2, we show the observed spectrum and smoothed spectrum in the Fe II, Ca II and He I* region. The dashed line is the power-law fit to the line-free regions, while the dot-dashed line is the smoothed result. The upper panel is the spectrum around Fe II lines and the lower panel is the spectrum around He I λ3889. The SDSS spectrum is divided by the smoothed spectrum to get the normalized spectrum, which is shown in Fig.3. The line-center positions of the identified Fe II* absorption lines together with He I and Ca II lines are marked by vertical dashed lines. The central wavelengths of Fe II* absorption lines and corresponding transitions are listed in Table 1, including mainly the absorption produced by the excited Fe II* at the wave numbers of 7955 cm−1 , 13474 cm−1 and 13674 cm−1 . We use the scaled He I* profile to fit the Fe II lines and measure their equivalent widths (EWs). Reasonable fitting can only be achieved for several Fe II lines because most lines are weak and suffer from blending, the Fe II column densities derived from rather strong lines are also listed in Table 1.
Fig. 2
Table 1 ˚) Wavelength(A 3184.035 3196.994 3214.237 3228.674 3244.659 3248.326 3256.826 3278.292
200 150 100 50 0 3100
3200
3300 Wavelength / Å
3400
3500
3700
3800 Wavelength / Å
3900
4000
120 100 80 60 40 20 0 3600
The fitted result of UV spectrum (3000 ∼ 4000 ˚ A)
Parameters of Fe II* absorption lines in SDSS J2220+0109 Transition a4 P3/2 -z4 F5/2 a4 P5/2 -z4 F7/2 a4 P3/2 -z4 D5/2 a4 P5/2 -z4 D7/2 c2 G9/2 -z2 F7/2 c2 G7/2 -z2 F7/2 a4 D7/2 -z6 D7/2 a4 D7/2 -z6 D9/2
Wave number(cm−1 ) 13673-45080 13474-44753 13673-44785 13474-44447 33466-64285 33501-64286 7955-38660 7955-38459
EW (˚ A) ... ... 1.27±0.15 1.37±0.14 ... ... ... 0.63±0.15
lgNion (cm−2 ) ... ... 14.37±0.05 14.30±0.04 ... ... ... 15.01±0.10
JI Tuo et al. / Chinese Astronomy and Astrophysics 37 (2013) 17–27
1.2 1.0
Fig. 3
3180
3200 3220 3240 Wavelength / Å
Fe II*3278.29
Fe II*3256.83
3260
3280
3300
1.2 1.0 0.6 0.4 0.2 0.0 3850
3900 3950 Wavelength / Å
Ca II3969.59
0.8 Ca II3934.78
Normalized flux
3160
Fe II*3244.66 Fe II*3248.33
0.2 0.0
Fe II*3228.67
0.4
Fe II*3214.24
Fe II*3184.03 He I3188.66
0.6
Fe II*3196.99
0.8
He I3889.74
Normalized flux
22
4000
Normalized spectrum in the UV band (3000 ∼ 4000 ˚ A)
2.3 Measurement Of Absorption Lines In the last subsection, we get the normalized Fe II spectrum directly from the division of the observed spectrum by the smoothed one, under the assumption that the absorption gases cover both the broad-line region (BLR) and continuum region. If this is not the case, the gases may only cover the continuum region but not the BLR, then we should subtract the broad line from the spectrum first, and divide the subtracted spectrum by the underlying power-law continuum to get the normalized spectrum. In Figure 4, we show both kinds of normalizations for the absorption lines of interest. The first normalization scheme is shown as solid lines with error bars, while the second is shown as dot-dashed lines. The two normalization schemes are almost indistinguishable for Hβ, He I*, Ca II and Fe II*λλ3214, 3228, because the fluxes of emission lines around these absorption lines are far less than that of the continuum. But the results differ significantly for the Hα absorption line because it sits on the top of the strong Hα emission line. In the panel of Hβ, we also show the expected Hα profile for the two kinds of normalizations. The direct normalization is shown as a dotted line, and the normalization with the broad line subtracted is shown as a dashed line. The grey horizontal lines mark the unit flux and the flux at the Hα line center to help the readers. To illustrate the profiles of weak lines clearly, we have contracted the y-axis range and shown only part of the Hα absorption line. Based on the result in Section 3.1, we measure the EW s of all the absorption lines using the profile of He I* λ3889 as done for Fe II*. Column densities are derived by assuming unsaturation. The result for non-Fe II lines are listed in Table 2. The fist column indicates the names of lines, the second column expresses the vacuum wavelengths, the third is for the EWs and the forth is for the column densities of corresponding species.
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Table 2 EWs of major absorption lines and column densities of corresponding ions in SDSS J2220+0109 Line Hα Hβ He I* He I* Ca II Ca II
˚) λ(A 6564.61 4862.68 3889.74 3188.66 3934.78 3969.59
EW (˚ A) 2.37±0.11 0.35±0.14 4.91±0.12 2.03±0.15 0.72±0.14 0.60±0.13
lgNion (cm−2 ) 12.98±0.02 13.14±0.17 15.11±0.01 15.31±0.03 12.91±0.08 13.13±0.09
1.1 0.8 0.5 0.2
HeI*3889.74
HeI*3188.66
FeII*3214.24
FeII*3228.67
CaII3934.78
CaII3969.59
Normalized flux
1.1 0.8 0.5 0.2 1.1 0.8 0.5 0.2 1.1 0.8 Hα6564.61 0.5 0.2 −1500 −1000 −500
Hβ4862.68
0 −1500 −1000
−500
0
Velocity / ( km⋅s−1)
Fig. 4
Two different normalization methods in velocity space
3. DISCUSSION 3.1 Position Of Absorption Gas The normalized flux of absorption lines can be expressed as I(v) = [1−Cf (v)]+Cf (v)× e−τ (v) , In which I(v) is the normalized velocity-dependent flux, Cf (v) is the line of sight covering factor, τ is the optical depth. If the absorption gas covers the background source, the ratio of optical depth between the absorption multiplets arising from the same energy level should be proportional to the ratio of their f × λ, in which f is the oscillator strength, and λ is the wavelength of the line. Taking the alkali-like ions such as Mg II and C IV for example, the strength ratio of strong to weak lines is 2:1; while for Balmer lines, Hα and Hβ have theoretical strength ratio of 7.26 : 1. We have plotted the predicted Hα profiles corresponding to the Hβ profiles in the two normalization schemes, as shown by the Hβ panel of Fig.4. The dotted line is the case when absorption gas fully covers the BLR and accretion disk, while the dashed line is the case that gas covers the accretion disk but not
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the BLR. In spite of the weakness of Hβ absorption line, the predicted profile using the first normalization scheme agrees well with the observed one (refer to the grey horizontal line). In addition, from Table 2 we get EW (Hα)/EW (Hβ)=6.77±2.73 and EW (He I λ3889)/EW (He I λ3189)=2.41±0.19, these values are in agreement with theoretical values. In contrast, if we adopt the second normalization scheme, the predicted Hα absorption is much deeper than the observed one. Thus we prefer the first normalization scheme, and use this scheme in the following analysis. Because of the low SNR of Hβ absorption line, it is difficult to determine whether the gas fully covers the BLR, but the observation of the strong He I* λ10830[15] and He I* λ3889 can be used to judge the existence of partial coverage, we have apply for the TripleSpec near-infrared spectroscopy to test it. In this paper, we assume that the gas fully covers 2 the background source, and as a first order approximation, we have N = πem2efcλ2 EW [26] . The column densities calculated in this way is tabulated in Tables 1∼2. When multiple lines arising from one and the same level are available, we always use the column density calculated from the strongest line in the following discussion. 3.2 Density Of Absorption Gas When thermal equilibrium is not achieved, the relative populations of excitation states to ground state of Fe II depend strongly on the gas density. In principle, one can measure the column densities of relevant levels of Fe II, then determine the gas density using the column density ratio of an excited level to ground level. However this method has only applied to several quasars. For example, Korista et al.[18] used the high-resolution Keck spectrum of Q2359-1241 to do such analysis. They detected the absorption lines of Fe II at ground and excited levels in the wavelength range of 2000∼3000 ˚ A(the UV1 multiplet near 2600 ˚ A and the UV2 multiplet near 2400 ˚ A), and based on the ratios of the numbers of ions at excited levels with the wave numbers of 385 cm−1 , 667 cm−1 , 862 cm−1 , 977 cm−1 and 7 955 cm−1 to the number of ions at the ground level, they estimated the density of absorption gas being about 103 cm−1 . The blue end of the spectrum of SDSS J2220+0109 only reaches 3100 ˚ A, no ground-level Fe II absorption lines fall in its spectrum range. However, we can still put constraints on the density via the absorption lines of the Fe II* ions excited from the levels with the wave numbers 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 , which are detected in its spectrum. If the electron density in the absorption gas is large enough and thermal equilibrium is [27] 2 established, we get nn21 = w from the Boltzmann equation, in which n1 and w1 exp(−χ/kT ) + n2 are the Fe densities of the ground level and one particular excited level respectively, w1 and w2 are the corresponding statistical weights, χ is the energy difference between the two levels involved. Substituting n2 with any two column densities among the levels of 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 (see Table 1), one can eliminate n1 and calculate the temperature needed for thermal equilibrium. The temperatures required for the thermal equilibrium between the level of 7.955 cm−1 and one of the levels of 13 474 cm−1 and 13 673 cm−1 are respectively (6 046±1 085) K and (10 541±3 476) K. From Ji et al.[10] and the following discussions, we know that Balmer absorption is probably related to the partially ionized region (PIZ). The temperature of PIZ is in the range 6 000∼104 K, depending on the shape of the incident ionizing continuum. Thus the temperatures needed are in agreement with those of PIZ. This in turn requires that the density is greater than the critical density
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nc = A21 /q21 . The critical densities of 7 955 cm−1 and 13 474 cm−1 levels are 105.9 cm−3 and 106.2 cm−3 respectively, thus the thermal equilibrium requires ne ≥ 106 cm−3 . In the following subsection, we know that the electron density can not exceed several 106 cm−3 to ensure that the gas fully covers the BLR. Put all these together, we know that the electron density should be around 106 cm−3 . The high-SNR spectrum and the observations of absorption lines of ground-level Fe II can help us put tighter constrains on the density. 3.3 The Origin Of Balmer Absorption Lines There are mainly three mechanisms to populate the hydrogen atom at the n=2 level: (1) collisional excitation, (2) recombination and (3) Lyα resonance scattering. Considered the resonance scattering of Lyα photons, the thermal equilibrium of hydrogen requires a lower density limit nc = A21 /[q21 (1 + τ0l )] = 8.7 × 1016 /(1 + τ0l ) cm−3 , where τ0l is the optical depth of Lyα resonance scattering. For the turbulent velocity of Gaussian distribution, we 3 can write τ0l 7.6 × 106 b−1 3 NH0 ,22 ∼ 10 , where b3 ∼ 1 is the turbulent velocity of gas in 3 −1 units of 10 km s , and NH0 ,22 is the neutral hydrogen column density in 1022 cm−2 . This gives a density too high for any reasonable line-absorbing gas. Thus we discuss the detailed equilibrium process in the following. The s-state and p-state of hydrogen at the n=2 level have different routes of radiative de-excitation: the 2s-state is de-excited via the forbidden 2-photon continuum process with a small Einstein coefficient A = 8.23 s−1 , while the de-excitation from 2p to 1s is allowed by emitting a Lyα photon. Therefore, we will consider them separately in the following analysis. The equilibrium for the 2p-level is determined by the competition between the recombination to n = 2 and the spontaneous radiation of Lyα photons1 . When the absorption gas is optically thick to Lyα photons, the equilibrium gives n2p /nH+ τ0l αeff ne /A2p,2s 7.3 × 10−9ne,6 NH0 ,22 b−1 3 , and the column density is inversely proportional to the square root of electron density: 1/2 −1/2
−1/2 b3 nH,6 (Nn=2 /1.4 × 1014 cm−2 )1/2 , NH0 ,22 ≈ 1.4fH−1 + (1 − fH+ )
(1)
where fH+ is fraction of ionized hydrogen, αeff is the effective recombination coefficient. It is very likely that the broad absorption line region (BALR) fully covers the BLR. This suggests rBALR > rBLR ∼ 1017 cm, where rBALR and rBLR are the sizes of broad absorption line region and broad emission line region, respectively. Substituting the observed value N (n = 2) ∼ 1013 cm−2 into Equation (1) , we infer a hydrogen column density of NH ∼ 1021 cm−2 of the partially ionized gas for an electron density ne of several 106 cm−3 . For a higher nH or a lower NH , the BALR would not be large enough to fully cover the BLR. Next, we consider the population of the 2s-level. The equilibrium equation for the 2s-level can be written as: ∞ αν Lν dν , (2) ne nH+ αeff,2s + nH ne q1s−2s = n2s A21 + ne (q2s−2p + q2s−nl ) + 2 ν1 4πr hν the first term on the left hand represents recombination to the 2s-level, while the second term represents collisional excitation. nl represents the atomic state with the energy level 1 As
we discuss next, the absorbing gas is probably in the partially ionized region, where the typical temperature is significantly lower than 104 K, and q1s−2p ∝ exp(− ΔE ) αeff , hence we ignore the kT collisional excitation of the 2p-level here.
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n and orbital angular momentum l. Given that αeff,2s and q1s−2s are of the same order of magnitude (αeff,2s = 0.838 × 10−13 cm−3 s−1 , q1s−2s = 1.67 × 10−13 cm−3 s−1 at T =10 000 K), the ratio of the first to second term is mainly determined by the ratio of nH+ to nH . In the H II region where nH+ /nH is high, the recombination mechanism dominates the population of n = 2 level, while in the H I region or PIZ where nH+ /nH is low, the collisional excitation dominates the population of n = 2 level. Ignoring unimportant terms[10] , in the H I region or PIZ with the temperature of about 4 10 K, we have n2s /nH ≤ 3 × 10−10 . The column density needed is about 3×1022 cm−2 , thus the collisional excitation is not as efficient as the Lyα resonance scattering; while in the H II region, the equilibrium gives n2s /nH+ ∼ α2s /q2s−2p ∼ 0.44 × 10−10 , the column density needed is 2×1023 cm−2 , thus the recombination mechanism is also less efficient than the mechanism of Lyα resonance scattering. Another piece of evidence that the recombination is not efficient enough comes from He I* absorption lines, which can be used to infer column density of H+ . These lines arise from the absorption of metastable 23 s-state He I*. The equilibrium equation for this level gives that: n23 s (He0 ) 5.8 × 10−6 T4−1.19 = , (3) nHe+ 1 + 3110T4−0.51n−1 e where T4 = T /104 K 1 for photoionized gas. At a reasonable gas density, the observed He I*(23 s) column density is 1.28× 1015 cm−2 , this gives the HII-region column density of 2.22× 1021 cm−2 , according to Equation (3). This column density is much lower than the column density 2×1023 cm−2 required for the excitation of n = 2 level via recombination, thus it is unlikely that recombination is the main mechanism for populating the n=2 hydrogen.
4. CONCLUSION AND FUTURE PERSPECTIVE In this paper, we analyze the unusual absorption line system in the quasar SDSS J2220+0109. The absorption lines include Hα, Hβ, He I* λλ3889, 3189, Ca II K, H, and multiple absorption lines from excited Fe II at states of the wave numbers 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 . From the population ratios of Fe II and the requirement that gases fully cover the BLR, the gas density is constrained to be ne ≈106 cm−3 . Based on the analysis on the observed optical depths of Balmer absorption multiplets, the gases should be outside the BLR and fully cover both the accretion disk and BLR. The unusual absorption line system in SDSS J2220+0109 hints that Balmer absorption lines could be closely related to Fe II* and He I* absorption lines. From Fe II* absorption lines, we can infer the gas density. Combining this with the photoionization model, one can put tight constraints on the geometry and the physical properties of the gases. SDSS J2220+0109 is the brightest Balmer absorption quasar reported so far, and it is very suitable for subsequent observations and campaigns. Beside the absorption lines, we find that the Fe II emission lines in SDSS J2220+0109 probably come from some low-density line-emitting regions. We are searching for similar quasars from the SDSS database, studies on these quasars will help us to understand the origin of Fe II emission lines in quasars.
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ELSEVIER
Chinese Astronomy and Astrophysics 37 (2013) 17–27
The Unusual Absorption Line Spectrum of Quasar SDSS J2220+0109 ZHOU Hong-yan1,2
JI Tuo1,2 1
WANG Ting-gui1
WANG Hui-yuan2
University of Science and Technology of China, Hefei 230026 2 Polar Research Institute of China, Shanghai 200136
Abstract We report the Balmer broad absorption lines (BALs) in the quasar SDSS J2220+0109 discovered from the SDSS data, and present a detailed analysis of the peculiar absorption line spectrum, including the He I* multiplet at λλ3189, 3889 arising from the metastable 23 s-state helium and the Balmer Hα and Hβ lines from the excited hydrogen H I of n=2 level, which are rarely seen in quasar spectra, as well as many absorption lines arising from the excited Fe II* of the levels 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 in the wavelength range 3100∼3300 ˚ A. Ca II H, K absorption line doublets also clearly appear in the SDSS spectrum. All absorption lines show a similar blueshifted velocity structure of Δv ≈ −1500 ∼ 0 km·s−1 relative to the quasar’s systematic redshift determined from the emission lines. Detailed analysis suggests that the Balmer absorption lines should arise from the partially ionized region with a column density of NH I ≈ 1021 cm−2 for an electron density of ne ∼ 106 cm−3 ; and that the hydrogen n=2 level may be populated via collisional excitation with Lyα pumping. In addition to its peculiar absorption features, SDSS J2220+0109 also shows unusual Fe II emission lines very likely originated from the low-density gas of ne ≈ 106 cm−3 . Key words: quasar: absorption lines—quasar: emission lines—quasar: individual: SDSS J2220+0109
Received 2011–12–13; revised version 2012–02–21 A translation of Acta Astron. Sin. Vol. 53, No.5, pp. 357–368, 2012 [email protected]
0275-1062/13/$-see front front mattermatter © 2013 Elsevier All rights reserved. c 2013 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2013.01.003 PII:
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1. INTRODUCTION Outflows in AGNs are potentially important for the co-evolution of black holes and their host galaxies. First, the outflowing gases carry away angular moment and facilitate the accretion of gas into black holes; Second, they influence the star formation and chemical evolution in host galaxies by funneling metal-enriched circum-nuclear gases into the interstellar even intergalactic space. Furthermore, in a longer time scale, outflows blow away nuclear gases and adjust the co-growth of black holes and galaxy bulges, this may explain their observed mass relation[1−3] . Outflows often show themselves as blueshifted absorption line systems in quasar spectra. Based on the width Δv of absorption lines, they mainly fall into 3 categories[4]: (1) broad absorption lines (BALs) with Δv ≥ 2000 km s−1 ; (2) narrow absorption lines (NALs) with Δv ≤ 500 km s−1 ; (3) mini-BALs with Δv in between. BALs are only seen in 15% of optically selected quasars. Based on the degree of ionization of absorption gases, BALs can be classified into 3 categories: (1) high-ionization broad absorption lines (HiBAL), i.e., the absorption lines arising generally from the absorption of C IV, Si IV and N V; (2) lowionization broad absorption-lines (LoBALs), about 15% of BAL quasars show absorption lines from low-ionization species such as Al III, Al II and Mg II, besides HiBALs; (3) iron low-ionization BALs (FeLoBALs), a minority (another 15%) of LoBAL quasars exhibit additional absorption of Fe II and Fe III at both ground and excited levels. Hydrogen Balmer absorption lines and He I* absorption lines are even rarer than FeLoBALs. The former arise from the absorption of hydrogen atoms excited from the n=2 level and are only detected in six quasars, including NGC 4151[5], SDSS J083942.11+380526.3[6], SDSS J125942.80+121312.6[7], SDSS J102839.11+450009.4[8], SDSS J172341.10+555340.5[9], and LBQS J1209+1633[10]. The latter arise from the absorption of photons by He I* of metastable 23 s-state. The population of He I* 23 s-level mainly comes from the recombination of He+ with electrons. Transitions from this level will generate a series of absorption lines with wavelengths ranging from near-infrared to ultraviolet, including He I* λλ10830, 3889, 3189, 2946, 2830, 2764. To our knowledge, He I* absorption lines are only detected in 5 quasars, including Mrk 231[11,12] , NGC 4151[5,13], Q 2359-1241[14], SDSS J102839.11+450009.42008 [8] and FBQS J1151+3822[15]. Both Balmer and He I* absorption lines are detected in NGC 4151 and SDSS J102839.11+450009.4, this may indicate that a certain relation may exist between the two kinds of lines in spite of the small sample sizes of the two. Another interesting fact is that two out of six Balmer absorption quasars show absorption lines arising from the excited Fe II at the same time. The non-detection of Fe II* absorption lines in the other two quasars may be due to either the low-SNR spectrum (SDSS J102839.11+450009.4) or the limited wavelength coverage (LBQS J1209+1633). It is probable that these two absorption phenomena are also closely related. If this is the case, these quasars will be valuable to the study of quasar outflows. Fe II* absorption lines can be used to determine the density of the absorption gas[16], while He I* absorption lines can help to understand the ionization state of the gas[14,17,18]. The combination of the two diagnostics can put constraints on the geometry and physical conditions of outflows. Discovery of more Balmer, He I* or Fe II* absorption line quasars can help to understand the relation between these absorption line systems and the possible origins of these systems. In this paper, we report the unusual absorption line quasar SDSS J222024.59+010931.2
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(hereafter SDSS J2220+0109) as reveled from the SDSS data. In its spectrum, we have detected simultaneously the absorption lines including Hα, Hβ, He I* λλ3889, 3189, Ca II H, K and Fe II* absorption lines arising from the levels of wave-numbers 7 955 cm−1 , 13474 cm−1 and 13673 cm−1 . We analyze the SDSS spectrum and measure the parameters of absorption lines in Section 2; we discuss the population mechanism of the absorption species and the physical conditions in the gas in Section 3; finally we discuss the implications of the discovery and make a summary in Section 4.
2. ANALYSIS OF SDSS SPECTRUM We searched systematically for the He I* and Balmer absorption lines in SDSS DR7 quasars and found about 200 quasars showing He I* absorption and about 20 showing Balmer absorption lines. SDSS J2220+0109 is the brightest among them with the I-band magnitude of (16.292±0.015) mag. The spectrum of this object was taken by SDSS 2.5 m telescope on 2001-08-19. The redshift re-measured on the peak of [O III] emission line is z=0.2122±0.0025. The spectrum was corrected for a Galactic extinction of E(B-V)=0.0632 and brought to the rest frame of the quasar before further analysis. The spectrum covers 3140∼7600 ˚ A in the rest frame. Balmer absorption lines sit on the top of Hα and Hβ emission lines, it makes their measurement depend on that of emission lines. On the other hand, the UV absorption lines are free of this problem. Thus different methods were adopted for the analysis of the optical spectrum and UV spectrum. 2.1 Optical Spectrum Because Balmer absorption lines are superposed on the broad Balmer emission lines, in order to obtain the normalized absorption spectrum and measure the parameters of absorption lines, the unabsorbed continuum and broad emission lines must be recovered at first. Generally, they are fitted by a power-law continuum and a template of broadened Fe II emission lines, in addition to one or more Gaussians for each emission line. Optical Fe II templates are usually built from the prototype narrow-line Seyfert 1 galaxy (NLS1) I Zw 1. The one built by Boroson et al.[19] (BG92) only includes broad Fe II lines. However, we note that there are obvious narrow emission lines near 5170 ˚ A in the spectrum of SDSS J2220+0109. Thus we adopt the Fe II template built by V´eron-Cetty et al. [20] (VJV04), which includes both broad and narrow Fe II emission lines. We use 4 Gaussians for the broad emission lines in the spectral range, including Hα, Hβ and Hγ; each narrow emission line in the spectral range is modeled with one Gaussian, and the redshifts of the narrow lines are assumed to be the same. We also fix the doublet ratio of [O III] λλ4959, 5007 to their theoretical value 1:3, the flux of narrow Hβ line is assumed to be 10% of the value of [O III] λ5007. All the above components are jointly used to fit the SDSS spectrum in the range of 4000∼7500 ˚ A, and the pixels affected by absorption lines are masked before fitting. The best-fit parameters are achieved by the minimization of χ2 . In the lower panel of Figure 1, we show the fitted result in the Hα region. The result agrees well with the SDSS spectrum. In the upper panel of Figure 1, we show the fitted result around Hβ and Hγ. The residual spectrum is shifted downward arbitrarily for clarity. There are clear emission peaks in the residual spectrum, especially on the red wing of Hβ emission line. The residual spectrum is very similar to the Fe II spectrum emitted by low-density (ne ≈ 106 cm−3 ) gas
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fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
as calculated by Bruhweiler et al. [21] The analysis of low-density Fe II emission is beyond the scope of this paper, thus we only show the calculated low-density Fe II spectrum for comparison in Figure 1.
Fig. 1
60 40 20 0 −20 4200
4400
4600
4800 5000 Wavelength / Å
5200
5400
5600
6600
6800
7000
200 SDSS spectrum
150
Best fit Residual
100
Bruhweiler’s model
50 0 −50 5600
5800
6000
6200 6400 Wavelength / Å
The fitted result of optical spectrum (4000∼7000 ˚ A)
Although the fitting of Hα region is satisfactory, the pollution of extra Fe II emission around Hβ and the weakness of the Hβ absorption line make the normalization of absorption lines very sensitive to the details of fitting. To normalize the spectrum in an unified way, we use a two-order polynomial to fit the Hα and Hβ locally. First, we use isolated He I* λλ3889, 3189 lines to determine the velocity range affected by absorption lines (Δv ≈ −1500 ∼ 0 km· s−1 ); then we mask the corresponding pixels of Hα and Hβ absorption lines and perform locally the two-order polynomial fitting. In the following analysis, we will use this fitted result for normalizing the spectrum. 2.2 Ultraviolet Spectrum In the UV band, besides the power-law continuum, the AGN spectrum is composed of the high-order Balmer lines, Balmer continuum and Fe II emission lines, which mix into a pseudo-continuum. The spectrum can in principle be decomposed using multi-component fitting. Two UV (<3500 ˚ A) Fe II templates are usually used in spectral decomposition, they are from Vestergaard et al.[22] and Tsuzuki et al.[23] respectively. But the blue end of the SDSS spectrum only reaches 3200 ˚ A, and there seems to be extra narrow Fe II emission [8] multiplet found by Wang et al. around 3100∼3200 ˚ A, which are not accounted for in the above mentioned Fe II templates. All these factors make the decomposition technique unreliable, hence we also recover the unabsorbed spectrum by means of local polynomial regression as done for Balmer lines. We still mask the pixels affected by absorption lines as done for Hα and Hβ. Then we linearly interpolate on the masked pixels and perform locally the weighted scatter-plot smoothing (LOWESS) [24,25] algorithm on the interpolated spectrum. In the LOWESS process, a low-order polynomial is fitted to a fixed width of the spectrum iteratively by means of weighted least square. In each succeed iteration, the weights are chosen to be inversely proportional to the n-th power of the deviation between
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fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
fλ / (10−17ergs⋅s−1⋅cm−2⋅Å−1)
the data and the currently fitted result. The iterations converge to our required absorptionfree spectrum. The algorithm is insensitive to narrow features including narrow emission and absorption lines and can maintain the broad features such as the Balmer continuum, highorder Balmer lines and Fe II emission lines. In Figure 2, we show the observed spectrum and smoothed spectrum in the Fe II, Ca II and He I* region. The dashed line is the power-law fit to the line-free regions, while the dot-dashed line is the smoothed result. The upper panel is the spectrum around Fe II lines and the lower panel is the spectrum around He I λ3889. The SDSS spectrum is divided by the smoothed spectrum to get the normalized spectrum, which is shown in Fig.3. The line-center positions of the identified Fe II* absorption lines together with He I and Ca II lines are marked by vertical dashed lines. The central wavelengths of Fe II* absorption lines and corresponding transitions are listed in Table 1, including mainly the absorption produced by the excited Fe II* at the wave numbers of 7955 cm−1 , 13474 cm−1 and 13674 cm−1 . We use the scaled He I* profile to fit the Fe II lines and measure their equivalent widths (EWs). Reasonable fitting can only be achieved for several Fe II lines because most lines are weak and suffer from blending, the Fe II column densities derived from rather strong lines are also listed in Table 1.
Fig. 2
Table 1 ˚) Wavelength(A 3184.035 3196.994 3214.237 3228.674 3244.659 3248.326 3256.826 3278.292
200 150 100 50 0 3100
3200
3300 Wavelength / Å
3400
3500
3700
3800 Wavelength / Å
3900
4000
120 100 80 60 40 20 0 3600
The fitted result of UV spectrum (3000 ∼ 4000 ˚ A)
Parameters of Fe II* absorption lines in SDSS J2220+0109 Transition a4 P3/2 -z4 F5/2 a4 P5/2 -z4 F7/2 a4 P3/2 -z4 D5/2 a4 P5/2 -z4 D7/2 c2 G9/2 -z2 F7/2 c2 G7/2 -z2 F7/2 a4 D7/2 -z6 D7/2 a4 D7/2 -z6 D9/2
Wave number(cm−1 ) 13673-45080 13474-44753 13673-44785 13474-44447 33466-64285 33501-64286 7955-38660 7955-38459
EW (˚ A) ... ... 1.27±0.15 1.37±0.14 ... ... ... 0.63±0.15
lgNion (cm−2 ) ... ... 14.37±0.05 14.30±0.04 ... ... ... 15.01±0.10
JI Tuo et al. / Chinese Astronomy and Astrophysics 37 (2013) 17–27
1.2 1.0
Fig. 3
3180
3200 3220 3240 Wavelength / Å
Fe II*3278.29
Fe II*3256.83
3260
3280
3300
1.2 1.0 0.6 0.4 0.2 0.0 3850
3900 3950 Wavelength / Å
Ca II3969.59
0.8 Ca II3934.78
Normalized flux
3160
Fe II*3244.66 Fe II*3248.33
0.2 0.0
Fe II*3228.67
0.4
Fe II*3214.24
Fe II*3184.03 He I3188.66
0.6
Fe II*3196.99
0.8
He I3889.74
Normalized flux
22
4000
Normalized spectrum in the UV band (3000 ∼ 4000 ˚ A)
2.3 Measurement Of Absorption Lines In the last subsection, we get the normalized Fe II spectrum directly from the division of the observed spectrum by the smoothed one, under the assumption that the absorption gases cover both the broad-line region (BLR) and continuum region. If this is not the case, the gases may only cover the continuum region but not the BLR, then we should subtract the broad line from the spectrum first, and divide the subtracted spectrum by the underlying power-law continuum to get the normalized spectrum. In Figure 4, we show both kinds of normalizations for the absorption lines of interest. The first normalization scheme is shown as solid lines with error bars, while the second is shown as dot-dashed lines. The two normalization schemes are almost indistinguishable for Hβ, He I*, Ca II and Fe II*λλ3214, 3228, because the fluxes of emission lines around these absorption lines are far less than that of the continuum. But the results differ significantly for the Hα absorption line because it sits on the top of the strong Hα emission line. In the panel of Hβ, we also show the expected Hα profile for the two kinds of normalizations. The direct normalization is shown as a dotted line, and the normalization with the broad line subtracted is shown as a dashed line. The grey horizontal lines mark the unit flux and the flux at the Hα line center to help the readers. To illustrate the profiles of weak lines clearly, we have contracted the y-axis range and shown only part of the Hα absorption line. Based on the result in Section 3.1, we measure the EW s of all the absorption lines using the profile of He I* λ3889 as done for Fe II*. Column densities are derived by assuming unsaturation. The result for non-Fe II lines are listed in Table 2. The fist column indicates the names of lines, the second column expresses the vacuum wavelengths, the third is for the EWs and the forth is for the column densities of corresponding species.
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Table 2 EWs of major absorption lines and column densities of corresponding ions in SDSS J2220+0109 Line Hα Hβ He I* He I* Ca II Ca II
˚) λ(A 6564.61 4862.68 3889.74 3188.66 3934.78 3969.59
EW (˚ A) 2.37±0.11 0.35±0.14 4.91±0.12 2.03±0.15 0.72±0.14 0.60±0.13
lgNion (cm−2 ) 12.98±0.02 13.14±0.17 15.11±0.01 15.31±0.03 12.91±0.08 13.13±0.09
1.1 0.8 0.5 0.2
HeI*3889.74
HeI*3188.66
FeII*3214.24
FeII*3228.67
CaII3934.78
CaII3969.59
Normalized flux
1.1 0.8 0.5 0.2 1.1 0.8 0.5 0.2 1.1 0.8 Hα6564.61 0.5 0.2 −1500 −1000 −500
Hβ4862.68
0 −1500 −1000
−500
0
Velocity / ( km⋅s−1)
Fig. 4
Two different normalization methods in velocity space
3. DISCUSSION 3.1 Position Of Absorption Gas The normalized flux of absorption lines can be expressed as I(v) = [1−Cf (v)]+Cf (v)× e−τ (v) , In which I(v) is the normalized velocity-dependent flux, Cf (v) is the line of sight covering factor, τ is the optical depth. If the absorption gas covers the background source, the ratio of optical depth between the absorption multiplets arising from the same energy level should be proportional to the ratio of their f × λ, in which f is the oscillator strength, and λ is the wavelength of the line. Taking the alkali-like ions such as Mg II and C IV for example, the strength ratio of strong to weak lines is 2:1; while for Balmer lines, Hα and Hβ have theoretical strength ratio of 7.26 : 1. We have plotted the predicted Hα profiles corresponding to the Hβ profiles in the two normalization schemes, as shown by the Hβ panel of Fig.4. The dotted line is the case when absorption gas fully covers the BLR and accretion disk, while the dashed line is the case that gas covers the accretion disk but not
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the BLR. In spite of the weakness of Hβ absorption line, the predicted profile using the first normalization scheme agrees well with the observed one (refer to the grey horizontal line). In addition, from Table 2 we get EW (Hα)/EW (Hβ)=6.77±2.73 and EW (He I λ3889)/EW (He I λ3189)=2.41±0.19, these values are in agreement with theoretical values. In contrast, if we adopt the second normalization scheme, the predicted Hα absorption is much deeper than the observed one. Thus we prefer the first normalization scheme, and use this scheme in the following analysis. Because of the low SNR of Hβ absorption line, it is difficult to determine whether the gas fully covers the BLR, but the observation of the strong He I* λ10830[15] and He I* λ3889 can be used to judge the existence of partial coverage, we have apply for the TripleSpec near-infrared spectroscopy to test it. In this paper, we assume that the gas fully covers 2 the background source, and as a first order approximation, we have N = πem2efcλ2 EW [26] . The column densities calculated in this way is tabulated in Tables 1∼2. When multiple lines arising from one and the same level are available, we always use the column density calculated from the strongest line in the following discussion. 3.2 Density Of Absorption Gas When thermal equilibrium is not achieved, the relative populations of excitation states to ground state of Fe II depend strongly on the gas density. In principle, one can measure the column densities of relevant levels of Fe II, then determine the gas density using the column density ratio of an excited level to ground level. However this method has only applied to several quasars. For example, Korista et al.[18] used the high-resolution Keck spectrum of Q2359-1241 to do such analysis. They detected the absorption lines of Fe II at ground and excited levels in the wavelength range of 2000∼3000 ˚ A(the UV1 multiplet near 2600 ˚ A and the UV2 multiplet near 2400 ˚ A), and based on the ratios of the numbers of ions at excited levels with the wave numbers of 385 cm−1 , 667 cm−1 , 862 cm−1 , 977 cm−1 and 7 955 cm−1 to the number of ions at the ground level, they estimated the density of absorption gas being about 103 cm−1 . The blue end of the spectrum of SDSS J2220+0109 only reaches 3100 ˚ A, no ground-level Fe II absorption lines fall in its spectrum range. However, we can still put constraints on the density via the absorption lines of the Fe II* ions excited from the levels with the wave numbers 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 , which are detected in its spectrum. If the electron density in the absorption gas is large enough and thermal equilibrium is [27] 2 established, we get nn21 = w from the Boltzmann equation, in which n1 and w1 exp(−χ/kT ) + n2 are the Fe densities of the ground level and one particular excited level respectively, w1 and w2 are the corresponding statistical weights, χ is the energy difference between the two levels involved. Substituting n2 with any two column densities among the levels of 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 (see Table 1), one can eliminate n1 and calculate the temperature needed for thermal equilibrium. The temperatures required for the thermal equilibrium between the level of 7.955 cm−1 and one of the levels of 13 474 cm−1 and 13 673 cm−1 are respectively (6 046±1 085) K and (10 541±3 476) K. From Ji et al.[10] and the following discussions, we know that Balmer absorption is probably related to the partially ionized region (PIZ). The temperature of PIZ is in the range 6 000∼104 K, depending on the shape of the incident ionizing continuum. Thus the temperatures needed are in agreement with those of PIZ. This in turn requires that the density is greater than the critical density
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nc = A21 /q21 . The critical densities of 7 955 cm−1 and 13 474 cm−1 levels are 105.9 cm−3 and 106.2 cm−3 respectively, thus the thermal equilibrium requires ne ≥ 106 cm−3 . In the following subsection, we know that the electron density can not exceed several 106 cm−3 to ensure that the gas fully covers the BLR. Put all these together, we know that the electron density should be around 106 cm−3 . The high-SNR spectrum and the observations of absorption lines of ground-level Fe II can help us put tighter constrains on the density. 3.3 The Origin Of Balmer Absorption Lines There are mainly three mechanisms to populate the hydrogen atom at the n=2 level: (1) collisional excitation, (2) recombination and (3) Lyα resonance scattering. Considered the resonance scattering of Lyα photons, the thermal equilibrium of hydrogen requires a lower density limit nc = A21 /[q21 (1 + τ0l )] = 8.7 × 1016 /(1 + τ0l ) cm−3 , where τ0l is the optical depth of Lyα resonance scattering. For the turbulent velocity of Gaussian distribution, we 3 can write τ0l 7.6 × 106 b−1 3 NH0 ,22 ∼ 10 , where b3 ∼ 1 is the turbulent velocity of gas in 3 −1 units of 10 km s , and NH0 ,22 is the neutral hydrogen column density in 1022 cm−2 . This gives a density too high for any reasonable line-absorbing gas. Thus we discuss the detailed equilibrium process in the following. The s-state and p-state of hydrogen at the n=2 level have different routes of radiative de-excitation: the 2s-state is de-excited via the forbidden 2-photon continuum process with a small Einstein coefficient A = 8.23 s−1 , while the de-excitation from 2p to 1s is allowed by emitting a Lyα photon. Therefore, we will consider them separately in the following analysis. The equilibrium for the 2p-level is determined by the competition between the recombination to n = 2 and the spontaneous radiation of Lyα photons1 . When the absorption gas is optically thick to Lyα photons, the equilibrium gives n2p /nH+ τ0l αeff ne /A2p,2s 7.3 × 10−9ne,6 NH0 ,22 b−1 3 , and the column density is inversely proportional to the square root of electron density: 1/2 −1/2
−1/2 b3 nH,6 (Nn=2 /1.4 × 1014 cm−2 )1/2 , NH0 ,22 ≈ 1.4fH−1 + (1 − fH+ )
(1)
where fH+ is fraction of ionized hydrogen, αeff is the effective recombination coefficient. It is very likely that the broad absorption line region (BALR) fully covers the BLR. This suggests rBALR > rBLR ∼ 1017 cm, where rBALR and rBLR are the sizes of broad absorption line region and broad emission line region, respectively. Substituting the observed value N (n = 2) ∼ 1013 cm−2 into Equation (1) , we infer a hydrogen column density of NH ∼ 1021 cm−2 of the partially ionized gas for an electron density ne of several 106 cm−3 . For a higher nH or a lower NH , the BALR would not be large enough to fully cover the BLR. Next, we consider the population of the 2s-level. The equilibrium equation for the 2s-level can be written as: ∞ αν Lν dν , (2) ne nH+ αeff,2s + nH ne q1s−2s = n2s A21 + ne (q2s−2p + q2s−nl ) + 2 ν1 4πr hν the first term on the left hand represents recombination to the 2s-level, while the second term represents collisional excitation. nl represents the atomic state with the energy level 1 As
we discuss next, the absorbing gas is probably in the partially ionized region, where the typical temperature is significantly lower than 104 K, and q1s−2p ∝ exp(− ΔE ) αeff , hence we ignore the kT collisional excitation of the 2p-level here.
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n and orbital angular momentum l. Given that αeff,2s and q1s−2s are of the same order of magnitude (αeff,2s = 0.838 × 10−13 cm−3 s−1 , q1s−2s = 1.67 × 10−13 cm−3 s−1 at T =10 000 K), the ratio of the first to second term is mainly determined by the ratio of nH+ to nH . In the H II region where nH+ /nH is high, the recombination mechanism dominates the population of n = 2 level, while in the H I region or PIZ where nH+ /nH is low, the collisional excitation dominates the population of n = 2 level. Ignoring unimportant terms[10] , in the H I region or PIZ with the temperature of about 4 10 K, we have n2s /nH ≤ 3 × 10−10 . The column density needed is about 3×1022 cm−2 , thus the collisional excitation is not as efficient as the Lyα resonance scattering; while in the H II region, the equilibrium gives n2s /nH+ ∼ α2s /q2s−2p ∼ 0.44 × 10−10 , the column density needed is 2×1023 cm−2 , thus the recombination mechanism is also less efficient than the mechanism of Lyα resonance scattering. Another piece of evidence that the recombination is not efficient enough comes from He I* absorption lines, which can be used to infer column density of H+ . These lines arise from the absorption of metastable 23 s-state He I*. The equilibrium equation for this level gives that: n23 s (He0 ) 5.8 × 10−6 T4−1.19 = , (3) nHe+ 1 + 3110T4−0.51n−1 e where T4 = T /104 K 1 for photoionized gas. At a reasonable gas density, the observed He I*(23 s) column density is 1.28× 1015 cm−2 , this gives the HII-region column density of 2.22× 1021 cm−2 , according to Equation (3). This column density is much lower than the column density 2×1023 cm−2 required for the excitation of n = 2 level via recombination, thus it is unlikely that recombination is the main mechanism for populating the n=2 hydrogen.
4. CONCLUSION AND FUTURE PERSPECTIVE In this paper, we analyze the unusual absorption line system in the quasar SDSS J2220+0109. The absorption lines include Hα, Hβ, He I* λλ3889, 3189, Ca II K, H, and multiple absorption lines from excited Fe II at states of the wave numbers 7 955 cm−1 , 13 474 cm−1 and 13 673 cm−1 . From the population ratios of Fe II and the requirement that gases fully cover the BLR, the gas density is constrained to be ne ≈106 cm−3 . Based on the analysis on the observed optical depths of Balmer absorption multiplets, the gases should be outside the BLR and fully cover both the accretion disk and BLR. The unusual absorption line system in SDSS J2220+0109 hints that Balmer absorption lines could be closely related to Fe II* and He I* absorption lines. From Fe II* absorption lines, we can infer the gas density. Combining this with the photoionization model, one can put tight constraints on the geometry and the physical properties of the gases. SDSS J2220+0109 is the brightest Balmer absorption quasar reported so far, and it is very suitable for subsequent observations and campaigns. Beside the absorption lines, we find that the Fe II emission lines in SDSS J2220+0109 probably come from some low-density line-emitting regions. We are searching for similar quasars from the SDSS database, studies on these quasars will help us to understand the origin of Fe II emission lines in quasars.
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